1. Field of the Invention
The present invention relates to a solder deposition method and a solder bump forming method that are suitable for surface mount of electronic parts on electronic circuit substrates.
2. Description of Related Art
In recent years, to meet the demand for smaller and lighter-weight of electronic equipment, multi-pin and narrow-pitch of electronic parts is developed, and fine pitch of conductor patterns is also advanced in which a large number of conductors are formed in a narrow range at very small intervals. Therefore, in place of conventional wire bonding, mounting method using solder bumps is widely employed to connect electronic parts to an electronic circuit substrate. Via-on-pad structure where via holes were connected with pads in a substrate for the sake of wiring design is also widely employed.
For solder bumps at fine pitch, it is required to keep the height of bumps constant with sufficient precision in order to connect securely electronic parts and obtain high reliability. Further, as shown in FIG. 2, for example, in a via-on-pad wherein a via hole 11 is provided in a substrate 10, the joint strength of electronic parts becomes low because of a void 13 within a solder bump 12 formed on an electrode 14. It is therefore also required that no void 13 exists within the solder bump 12.
Instead of conventional plating and deposition, metal mask method and resin mask method, each using a solder paste that is a mixture of solder powder and flux, are widely employed as a method of forming solder bumps.
However, Japanese Patent Unexamined Publication No. 2002-334895 describes that metal mask method has poor precision in bump height, and that resin mask method suffers from the drawback that the electronic part assembly is obstructed by a part of the resin film residue on the substrate surface (see columns [0005] to [0009] in this publication).
In Publication No. 2002-334895, to eliminate the drawback of resin mask method, the curing of a resin film is suppressed so that the resin film is readily removed from the substrate surface. In accordance with the conventional solder bump forming method using a solder paste, however, a void is apt to occur within bumps, and therefore joint strength lowers, thus failing to obtain high reliability. Particularly when using a via-on-pad structured substrate, or when using a lead-free solder paste containing no lead in solder material, a void is apt to occur.
On the other hand, as other method of forming fine pitch solder bumps, there is known a method wherein a solder precipitating composition containing tin powder and lead salt of organic acid is applied on all over a substrate surface including electrodes, and then heated to separate metal lead, thereby depositing solder alloy on the surface of the electrode (Japanese Patent Unexamined Publication No. 1-157796). That is, when tin powder and lead salt of organic acid are mixed and heated, lead atoms of the lead salt of organic acid are replaced by tin atoms and then separated, which thereafter diffuse into excess tin metal powder and form Sn—Pb alloy. Recently, the development of lead-free solder precipitating compositions is also advanced from the demand for lead-free.
The use of such solder precipitating composition enables to form bumps precisely on electrodes of fine pitch substrates, and also prevent occurrence of voids as in the case of using a solder paste. With a method of forming bumps using a solder precipitating composition, it is however difficult to form bumps having a desired height in a single deposition operation. This method therefore often requires at least two deposition operations.